PERCENTAGE COMPOSITION Just like any percentage problems you are comparing the part to the whole. In chemistry, percentage composition is based on mass, not on numbers of atoms present. For Example, if I have 5 pieces of fruit in a dish and 2 of the pieces of fruit are bananas, what percentage of the fruit in the dish is bananas? We are going to find the percentage BY MASS of certain elements in a compound by comparing the mass of the element to the mass of the entire compound. STEPS: To find percentage composition: 1. write a correct formula for the compound in the question 2. find the molar mass of the entire compound 3. divide the total mass of the element whose percentage you are looking for by the total mass of the entire compound. (Multiply the result by 100 to convert it to %) 4. express your answer to 1 decimal place and add the % sign as the label Let s try a few examples: 1) sodium bromide % Na % Br 2) aluminum acetate % Al % C % H % O 3) ammonium sulfite % N % H % S % O
Percent by Mass of Sugar in Gum 1. Mass a piece of gum and record it in the data table. 2. Remove the sugar from the gum (hint: sugar is soluble in water and saliva) 3. Measure the mass of the gum with no sugar and record it in the data table. 4. Calculate the mass of the sugar that you removed and record. 5. Calculate the percent of the original mass that was sugar and record. 6. Use the nutrition facts label to calculate the percent sugar by mass that is reported by the manufacturer, and record that value on your data table. 7. Compare the reported value with yours circle the greater percentage in your data table. 8. Calculate the number of molecules of sugar (C12H22O11) and record it in your data table. original mass mass with no sugar mass of sugar removed percent sugar by mass mfr. reported percent sugar molecules of sugar Space for calculations: Homework: Percent Composition Calculate the percent by mass of the following: carbon and hydrogen in propane aluminum and chlorine in aluminum chloride sulfur and oxygen in sulfur dioxide carbon, hydrogen, and oxygen in C 6H 12O 6 barium, nitrogen, and oxygen in barium nitrate hydrogen, sulfur, and oxygen in sulfuric acid
Empirical Formula vs Molecular Formula An empirical formula is one which is written in its very lowest terms - it does not necessarily give the exact number of atoms of everything present, but rather the RATIO of the atoms present.) For example, C6H12O6 has six carbons, twelve hydrogens and six oxygens. So we could write the formula like this: C1H2O1 (or CH2O). Contrast the above definition to this one for molecular formula: the formula s subscripts give the actual number of each element in the formula C6H12O6 is the molecular formula because it shows exactly how many atoms of each element are present Here are some examples: Molecular Formula Empirical Formula C6H12O6 CH3COOH CH2O CH2O N2O6 C4H8 NaNO3 NO3 CH2 NaNO3 Notice three things: 1. The molecular formula and the empirical formula can be identical. 2. Some compounds can have the same empirical formula but different molecular formula. 3. You scale up from the empirical formula to the molecular formula by a whole number factor. what whole numbers apply to the chart above? 2 types of problems need to be solved with empirical formulas. 1. Using percent composition to find empirical formula 2. Using Empirical formula and molar mass to find molecular formula
1. Using percent composition to find empirical formula Here s a basic rhyme that might help: Percent to mass Mass to mole Divide by small Multiply 'til whole Here's an example of how it works: Problem: A compound consists of 72.2% magnesium and 27.8% nitrogen. Find the empirical formula. (1) Percent to mass: Assume 100 g of the substance, so (2) Mass to moles: 1-step conversions: (3) Divide by small: make sure you divide ALL answers from #2 by the smallest value. (4) Multiply 'til whole: multiply ALL values from #3 by the same factor. This factor is selected so as to produce ALL whole numbers as answers. Often this factor is chosen by trial-and-error. PRACTICE: What s the empirical formula of a molecule containing 18.7% lithium, 16.3% carbon, and 65.0% oxygen?
2. Using Empirical formula and molar mass to find molecular formula Here's an example of how it works. A compound with an empirical formula of C4H4O has a molar mass of 136 g/mol: 1) Calculate the "empirical formula mass : add up the molar mass of the empirical formula. 2) Divide the molar mass by the empirical formula mass to get a whole number factor 3) Multiply the subscripts of the empirical formula by the factor just computed. Practice: What is the molecular formula of a compound with an empirical formula of C2H8N and a molar mass of 138 g/mol?